Sheet thickness measurement

Abstract

A method of measuring a thickness of a sheet being conveyed on a transport path includes rotating a substantially cylindrical reference surface disposed in the transport path and engaging a probe with the reference surface to determine a runout value for each of a set of positions along a circumference of the reference surface. The method further includes conveying the sheet on the transport path so that the sheet contacts the reference surface at one position of the set, engaging the probe with the sheet at the one position to determine a measured sheet thickness value, and adjusting the measured sheet thickness value based on the runout value for the one position to obtain an actual sheet thickness value.

1. A method of measuring a thickness of a sheet being conveyed on a transport path, comprising:
 rotating a substantially cylindrical reference surface disposed in the transport path; 
 engaging a probe with the reference surface to determine a runout value for each of a set of positions along a circumference of the reference surface; 
 controlling the rotation of the reference surface and engagement of the probe by a controller operatively connected to a processing device; 
 conveying the sheet on the transport path so that the sheet contacts the reference surface at one position of the set; 
 engaging the probe with the sheet at the one position to determine a measured sheet thickness value; 
 storing the runout value for each of the set of positions along the circumference of the reference surface in a database on the processing device; and 
 adjusting the measured sheet thickness value based on the runout value for the one position to obtain an actual sheet thickness value by adding the runout value to the measured sheet thickness where the runout value is negative; and subtracting the runout value from the measured sheet thickness where the runout value is positive. 
 
     
     
       2. The method of  claim 1 , wherein the sheet is conveyed at a transport speed, and wherein the substantially cylindrical reference surface is rotated approximately at the transport speed. 
     
     
       3. The method of  claim 1 , wherein the substantially cylindrical reference surface is rotated by a first actuator provided with a first positional encoder to track the position of the reference surface. 
     
     
       4. The method of  claim 3 , wherein the probe is driven for engagement with the reference surface and the sheet by a second actuator provided with a second positional encoder to track a position of the probe. 
     
     
       5. The method of  claim 4 , wherein the probe comprises:
 a support element operatively connected to the second actuator; and 
 a rotatably mounted probe tip disposed on the support element. 
 
     
     
       6. The method of  claim 5 , wherein the probe tip is configured to contact the reference surface and the sheet in rolling engagement. 
     
     
       7. The method of  claim 4 , wherein the reference surface and the second actuator are disposed on a substantially rigid frame assembly. 
     
     
       8. The method of  claim 4 , wherein the second actuator is selectively displaceable axially with respect to the reference surface. 
     
     
       9. The method of  claim 1 , further comprising:
 conveying the sheet on the transport path so that the sheet contacts the reference surface at a plurality of positions of the set; 
 engaging the probe with the sheet at the plurality of positions to determine a measured sheet thickness value for each of the positions; and 
 adjusting each measured sheet thickness value based on the runout value for each of the positions to obtain an actual sheet thickness value for each of the positions. 
 
     
     
       10. The method of  claim 1 , further comprising:
 re-engaging the probe with the reference surface to determine an updated runout value for each of the set of positions along the circumference of the reference surface; 
 storing the updated runout value for each of the set of positions along the circumference of the reference surface in the database; 
 comparing each runout value with a corresponding updated runout value to determine a difference for each position; and 
 carrying out an action when the differences for selected positions exceed a predetermined level. 
 
     
     
       11. A method of measuring a thickness of a sheet being conveyed on a transport path at a transport speed, comprising:
 rotating a substantially cylindrical reference surface disposed in the transport path approximately at the transport speed; 
 engaging a probe with the reference surface to determine a runout value for each of a set of positions along a circumference of the reference surface; 
 conveying the sheet on the transport path so that the sheet contacts the reference surface at a plurality of positions of the set; 
 engaging the probe with the sheet at the plurality of positions to determine a measured sheet thickness value for each of the positions; and 
 adjusting each measured sheet thickness value based on the runout value for each of the positions to obtain an actual sheet thickness value for each of the positions by adding the runout value to the measured sheet thickness where the runout value is negative; and subtracting the runout value from the measured sheet thickness where the runout value is positive. 
 
     
     
       12. The method of  claim 11 , wherein the substantially cylindrical reference surface is rotated by a first actuator provided with a first positional encoder to track the position of the reference surface, and wherein the probe is driven for engagement with the reference surface and the sheet by a second actuator provided with a second positional encoder to track a position of the probe. 
     
     
       13. The method of  claim 12 , wherein the probe comprises:
 a support element operatively connected to the second actuator; and 
 a rotatably mounted probe tip disposed on the support element configured to contact the reference surface and the sheet in rolling engagement. 
 
     
     
       14. The method of  claim 12 , wherein the reference surface and the second actuator are disposed on a substantially rigid frame assembly. 
     
     
       15. The method of  claim 12 , wherein the second actuator is selectively displaceable axially with respect to the reference surface. 
     
     
       16. The method of  claim 11 , further comprising:
 re-engaging the probe with the reference surface to determine an updated runout value for each of the set of positions along the circumference of the reference surface; 
 comparing each runout value with a corresponding updated runout value to determine a difference for each position; and 
 carrying out an action when the differences for selected positions exceed a predetermined level.